Sn ALLOY PLATING APPARATUS AND METHOD

ABSTRACT

An Sn alloy plating apparatus includes: a plating bath having a cathode chamber for holding therein an Sn alloy plating solution in which the substrate is to be immersed and an anode chamber for holding therein an anolyte containing Sn ions and an acid; an Sn anode located in the anode chamber; and an electrolytic solution supply line configured to supply an electrolytic solution containing the acid into the anode chamber such that a Sn ion concentration of the anolyte in the anode chamber is kept not less than a predetermined value and a concentration of the acid in the anolyte is kept not less than a predetermined acceptable value. The electrolytic solution supply line supplies the electrolytic solution into the anode chamber to increase an amount of the anolyte in the anode chamber and supply the anolyte into the Sn alloy plating solution by the increased amount.

CROSS-REFERENCE TO RELATED APPLICATION

This document claims priority to Japanese Patent Application No.2012-272168 filed Dec. 13, 2012, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an Sn alloy plating apparatus andmethod useful for forming a metal film of an alloy of Sn and a metalwhich is nobler than Sn (e.g., a lead-free Sn—Ag alloy having goodsoldering properties) on a substrate surface.

2. Description of the Related Art

As is known in the art, a plating film of an alloy of Sn (tin) and ametal which is nobler than Sn (e.g., an Sn—Ag alloy which is an alloy ofSn and silver), formed by electroplating on a substrate surface, can beused for lead-free solder bumps. Sn—Ag alloy plating is typicallycarried out by applying a voltage between an anode and a substratesurface, which are disposed opposite to each other and immersed in anSn—Ag alloy plating solution containing Sn ions and Ag ions, therebyforming an Sn—Ag alloy film on the substrate surface. Other than theSn—Ag alloy, an Sn—Cu alloy which is an alloy of Sn and Cu (copper), anSn—Bi alloy which is an alloy of Sn and Bi (bismuth), and the like canbe used as an alloy of Sn and a metal which is nobler than Sn.

An insoluble anode is often used in plating of such an alloy of Sn and ametal which is nobler than Sn. This is because, if a soluble anode madeof Sn (i.e., Sn anode) is used, displacement deposition of the noblermetal on the surface of the Sn anode will occur, leading to unstableconcentration of metal component and contamination of the platingsolution.

Various Sn alloy plating apparatuses and methods using a soluble anodemade of Sn (Sn anode) have been proposed. For example, a plating methodhas been proposed which involves separating an anode chamber, in whichan Sn anode is disposed, from a plating bath by using an anion exchangemembrane, and putting an Sn plating solution and an acid or a saltthereof into the anode chamber and putting an Sn alloy plating solutioninto the plating bath (see Japanese Patent No. 4441725). The Snion-containing solution in the anode chamber can be supplied through asupply line to the Sn alloy plating solution in the plating bath. Aplating method has been proposed which comprises carrying out plating ofa plating object in a plating bath by using an Sn anode which isisolated by an anode bag or box formed of a cation exchange membrane(see Japanese Patent No. 3368860).

Further, an Sn—Ag alloy plating method has been proposed which involvesproviding a plating bath with an auxiliary cell, having a cathodechamber and an anode chamber which are separated by a diaphragm so thata substance that can cause deterioration of a plating solution will notdiffuse into the cathode chamber, and supplying Sn ions to the platingsolution (anolyte) in the anode chamber in the auxiliary bath (seeJapanese Patent Laid-Open Publication No. H11-21692).

The abovementioned Japanese Patent No. 4441725 describes the methodincluding the steps of separating an anode chamber and a cathode chamberby an anion exchange membrane, putting an Sn anode into an electrolyticsolution (anolyte) containing Sn ions and an acid or a salt thereof,held in the anode chamber, to allow dissolution of Sn ions from the Snanode into the anolyte, and supplying the Sn ion in the anode chamber tothe cathode chamber. In this method, it has been found by the presentinventors that it is important to control the acid concentration of theanolyte in the anode chamber in order to achieve stable dissolution ofSn ions into the anolyte in the anode chamber. The method described inthis Japanese Patent No. 4441725 necessitates a supply line and a supplydevice, such as a pump, to supply the Sn ion to the cathode chamber,leading to a complicated construction of the apparatus.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above situation. Itis therefore an object of the present invention to provide an Sn alloyplating apparatus and method which appropriately control a concentrationof Sn ions and a concentration of an acid that forms a complex with adivalent Sn ion in an anolyte to be supplied to an Sn alloy platingsolution, to thereby enable relatively easy control of the Sn alloyplating solution and simplified construction of the apparatus.

An Sn alloy plating apparatus for electrodepositing an An Sn alloyplating apparatus for electrodepositing an alloy of Sn and a metal whichis nobler than Sn on a surface of a substrate is provided. The apparatuscomprises: a plating bath whose interior is separated by an anionexchange membrane into a cathode chamber for holding therein an Sn alloyplating solution in which the substrate, serving as a cathode, is to beimmersed and an anode chamber for holding therein an anolyte containingSn ions and an acid that forms a complex with a divalent Sn ion; an Snanode located in the anode chamber; and an electrolytic solution supplyline configured to supply an electrolytic solution containing the acidinto the anode chamber such that a Sn ion concentration of the anolytein the anode chamber is kept not less than a predetermined value and aconcentration of the acid in the anolyte is kept not less than apredetermined acceptable value, the electrolytic solution supply linebeing configured to supply the electrolytic solution into the anodechamber to increase an amount of the anolyte in the anode chamber andsupply the anolyte into the Sn alloy plating solution by the increasedamount.

According to the apparatus as described above, the Sn ion concentrationof the anolyte and the concentration of the acid that forms a complexwith a divalent Sn ion are controlled appropriately, the anolyte, havinga high Sn ion concentration and in which divalent Sn ions exist stably,is supplied to the Sn alloy plating solution. Therefore, it is possibleto supply Sn ions to the Sn alloy plating solution stably.

In an embodiment, the electrolytic solution supply line is configured tosupply the electrolytic solution into the anode chamber to increase anamount of the anolyte in the anode chamber to thereby cause the anolyteto overflow the anode chamber into the Sn alloy plating solution.

According to this embodiment, the anolyte, having a high Sn ionconcentration and in which divalent Sn ions exist stably, can besupplied to the Sn alloy plating solution without use of any power.

In an embodiment, the Sn alloy plating apparatus further includes: anoverflow bath configured to store the Sn alloy plating solution that hasoverflowed the cathode chamber; and a plating solution circulation lineconfigured to return the Sn alloy plating solution in the overflow bathto the cathode chamber to thereby circulate the Sn alloy platingsolution.

According to this embodiment, the Sn alloy plating solution in thecathode chamber circulates through the plating solution circulationline, so that the plating solution can be agitated.

In an embodiment, the Sn alloy plating apparatus further includes a purewater supply line configured to supply pure water into the anodechamber.

By adjusting the amount of pure water to be supplied through the purewater supply line into the anode chamber or the amount of theelectrolytic solution to be supplied through the electrolytic solutionsupply line into the anode chamber, the concentration of the acid in theanolyte can be controlled in a preferable range.

In an embodiment, the Sn alloy plating apparatus further comprises anacid concentration measuring device for measuring the concentration ofthe acid in the anolyte in the anode chamber.

In an embodiment, the Sn alloy plating apparatus further comprises adialysis cell configured to draw out a part of the Sn alloy platingsolution from the cathode chamber, remove at least a part of the acidfrom the Sn alloy plating solution, and then return the Sn alloy platingsolution to the cathode chamber.

When the concentration of the acid in the Sn alloy plating solution istoo high, at least a part of the acid can be removed from the Sn alloyplating solution by the dialysis cell so as to adjust the acidconcentration to a preferable range.

In an embodiment, the Sn alloy plating apparatus further comprises an N₂gas supply line configured to supply nitrogen gas into the anolyte inthe anode chamber to form nitrogen gas bubbles in the anolyte.

According to this embodiment, the anolyte in the anode chamber can besufficiently agitated with the bubbles of the nitrogen gas, so that Snions and the acid can be uniformly distributed in the anolyte. Inaddition, the bubbles of the nitrogen gas can prevent oxidation of theSn ions in the anolyte.

In an embodiment, the Sn alloy plating apparatus further comprises anauxiliary electrolytic cell configured to supply an anolyte having anincreased concentration of Sn ions to the Sn alloy plating solution. Theauxiliary electrolytic cell includes an auxiliary anode chamber forholding an anolyte therein, an auxiliary cathode chamber for holding acatholyte therein, an anion exchange membrane separating the auxiliaryanode chamber and the auxiliary cathode chamber from each other, anauxiliary Sn anode located in the auxiliary anode chamber, an auxiliarycathode located in the auxiliary cathode chamber, and an auxiliary powersource configured to apply a voltage between the auxiliary Sn anode andthe auxiliary cathode when the auxiliary Sn anode is immersed in theanolyte and the auxiliary cathode is immersed in the catholyte toproduce the anolyte having the increased concentration of Sn ions.

In the event of a shortage of Sn ions in the entire system, the shortagecan be compensated for by the supply of the anolyte, having a high Snion concentration, from the auxiliary anode chamber.

An Sn alloy plating method of electrodepositing an alloy of Sn and ametal which is nobler than Sn on a surface of a substrate, the methodcomprising: providing a plating bath whose interior is separated by ananion exchange membrane into a cathode chamber and an anode chamber;supplying an Sn alloy plating solution into the cathode chamber;immersing the substrate in the Sn alloy plating solution; supplying ananolyte, containing Sn ions and an acid that forms a complex with adivalent Sn ion, into the anode chamber to immerse an Sn anode in theanolyte; supplying an electrolytic solution containing the acid into theanode chamber such that a Sn ion concentration of the anolyte in theanode chamber is kept not less than a predetermined value and aconcentration of the acid in the anolyte is kept not less than apredetermined acceptable value; and applying a voltage between the Snanode and the substrate serving as a cathode to plate the surface of thesubstrate, while supplying the electrolytic solution into the anodechamber to increase an amount of the anolyte in the anode chamber andsupplying the anolyte into the Sn alloy plating solution by theincreased amount.

In an embodiment, the supplying the electrolytic solution into the anodechamber to increase an amount of the anolyte in the anode chamber andthe supplying the anolyte into the Sn alloy plating solution by theincreased amount comprises supplying the electrolytic solution into theanode chamber to increase an amount of the anolyte in the anode chamberto thereby cause the anolyte to overflow the anode chamber into the Snalloy plating solution.

In an embodiment, the Sn alloy plating method further includescirculating the Sn alloy plating solution in the cathode chamber.

In an embodiment, the Sn alloy plating method further includescontrolling an amount of the electrolytic solution or pure water to besupplied into the anode chamber based on the concentration of the acidin the anolyte held in the anode chamber.

In an embodiment, the Sn alloy plating method further includesdetermining the concentration of the acid in the anolyte from an initialacid concentration of the anolyte, a quantity of electricity and acurrent efficiency at the Sn anode, an amount of the electrolyticsolution supplied, and a permeability of the anion exchange membranewith respect to methanesulfonic acid that passes through the anionexchange membrane and migrates from the cathode chamber into the anodechamber.

In an embodiment, the Sn alloy plating method further includes drawingout a part of the Sn alloy plating solution from the cathode chamber;removing at least a part of the acid from the Sn alloy plating solutionthat has been drawn out; and then returning the Sn alloy platingsolution to the cathode chamber.

In an embodiment, the Sn alloy plating method further includes supplyingnitrogen gas into the anolyte in the anode chamber to form nitrogen gasbubbles in the anolyte.

In an embodiment, the Sn alloy plating method further includes immersingan auxiliary Sn anode in an anolyte held in an auxiliary anode chamber;immersing an auxiliary cathode in a catholyte held in an auxiliarycathode chamber that is separated from the auxiliary anode chamber by ananion exchange membrane; applying a voltage between the auxiliary Snanode and the auxiliary cathode to produce the anolyte having anincreased concentration of Sn ions; and supplying the anolyte having theincreased concentration of Sn ions into the Sn alloy plating solution.

According to the present invention, the electrolytic solution containingthe acid that forms a complex with a divalent Sn ion is supplied intothe anode chamber so that the Sn ion concentration of the anolyte in theanode chamber is kept not less than a predetermined value and theconcentration of the acid does not become lower than an acceptablevalue. The concentration of Sn ions and the concentration of the acid inthe anolyte can thus be appropriately controlled. Further, the anolyte,whose amount has been increased by the supply of the electrolyticsolution, in the anode chamber is supplied to the Sn alloy platingsolution. Thus, the anolyte, having a high Sn ion concentration and inwhich divalent Sn ions exist stably, is supplied to the Sn alloy platingsolution. This makes it possible to stably replenish the Sn alloyplating solution with Sn ions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an Sn alloy plating apparatus according toan embodiment;

FIG. 2 is a perspective view of an exemplary anode bath configured tocause an anolyte to overflow the bath;

FIG. 3 is a cross-sectional view of a main portion of another exemplaryanode bath configured to cause an anolyte to overflow the bath;

FIG. 4 is a perspective view of a main portion of yet another exemplaryanode bath configured to cause an anolyte to overflow the bath;

FIG. 5 is a schematic perspective view of a substrate holder shown inFIG. 1;

FIG. 6 is a plan view of the substrate holder shown in FIG. 1;

FIG. 7 is a right side view of the substrate holder shown in FIG. 1;

FIG. 8 is an enlarged view of the portion A of FIG. 7;

FIG. 9 is a diagram illustrating a main portion of the Sn alloy platingapparatus when performing the plating process;

FIG. 10 is a graph showing a theoretical Sn ion concentration of ananolyte in an anode chamber, calculated from a quantity of electricity,in comparison with actually measured Sn ion concentration of theanolyte;

FIG. 11 is a schematic view of another example of a plating bath;

FIG. 12 is a schematic view of an Sn alloy plating apparatus accordingto another embodiment;

FIG. 13 is a schematic view of an Sn alloy plating apparatus accordingto yet another embodiment;

FIG. 14 is a schematic view of an Sn alloy plating apparatus accordingto yet another embodiment; and

FIG. 15 is a schematic view of an Sn alloy plating apparatus accordingto yet another embodiment.

DETAILED DESCRIPTION

Embodiments will now be described in detail with reference to thedrawings. The same reference numerals are used in the figures anddescriptions to refer to the same or like members, components, etc., andduplicate descriptions thereof are omitted.

The following embodiment illustrates an exemplary case where Ag (silver)is used as a metal which is nobler than Sn (tin) and a film of an Sn—Agalloy is formed on a substrate surface. Methanesulfonic acid is used asan acid that forms a complex with a divalent Sn ion. Thus, an Sn—Agalloy plating solution is used which contains tin methanesulfonate as asource of Sn ions (Sn²⁺) and silver methanesulfonate as a source of Agions (Ag⁺). It is also possible to use silver alkylsulfonate as a sourceof Ag ions (Ag⁺).

FIG. 1 is a schematic view of an Sn alloy plating apparatus according toan embodiment. As shown in FIG. 1, the Sn alloy plating apparatusincludes a plating bath 16 in which a box-shaped anode bath 10 isdisposed. The interior of the plating bath 16 is divided by the anodebath 10 into a cathode chamber 12 and an anode chamber 14 which isdefined in the anode bath 10.

The cathode chamber 12 is coupled via an overflow bath 36, which will bedescribed later, to a plating solution supply line 20 extending from aplating solution supply source 18. The cathode chamber 12 is configuredto hold an Sn—Ag alloy plating solution (hereinafter referred to simplyas a plating solution) Q therein. A substrate W, which is detachablyheld by a substrate holder 22 and serves as a cathode during platingthereof, is put at a predetermined position in the cathode chamber 12and immersed in the plating solution Q when plating of the substrate Wis performed.

An anolyte supply line 23, an electrolytic solution supply line 24, apure water supply line 26, and a liquid discharge line 28 are coupled tothe anode chamber 14. The anode chamber 14 is configured to hold ananolyte E therein. A soluble Sn anode 32, which is made of Sn and heldby an anode holder 30, is disposed at a predetermined position in theanode chamber 14 and immersed in the anolyte E. Further, an N₂ gassupply line 33 for supplying nitrogen gas into the anolyte E to formnitrogen gas bubbles in the anolyte E is provided at a bottom of theanode chamber 14.

In this embodiment, a solution, containing Sn ions and methanesulfonicacid that forms a complex with a divalent Sn ion and not containing Agions, is used as the anolyte E. A part of methanesulfonate ions in theanolyte E surrounds the divalent Sn ion to form the complex with the Snion, while the other part of methanesulfonate ions exists as a free acidin the anolyte E. The methanesulfonic acid concentration herein refersto the concentration of the free acid unless otherwise stated. Becauseof the absence of Ag ions in the anolyte E, there is no possibility of areaction between Ag ions and the Sn anode 32, and a consequentdisplacement deposition of Ag on the surface of the Sn anode 32 does notoccur when the Sn anode 32 is immersed in the anolyte E. An aqueoussolution containing methanesulfonic acid (i.e., an aqueousmethanesulfonic acid solution) is used as the electrolytic solutionwhich is supplied into the anode chamber 14 through the electrolyticsolution supply line 24.

When carrying out plating of the substrate W, the Sn anode 32 iselectrically connected to a positive pole of a plating power source 34,and a conductive layer (not shown), such as a seed layer, formed on thesurface of the substrate W is electrically connected to a negative poleof the plating power source 34. As a result, a metal film of an Sn—Agalloy is formed on the surface of the conductive layer. This metal filmmay be used for lead-free solder bumps.

The plating bath 16 is provided with the overflow bath 36 which islocated adjacent to the cathode chamber 12. The plating solution Q isallowed to overflow the top of the cathode chamber 12 into the overflowbath 36. One end of a plating solution circulation line 46 is coupled tothe bottom of the overflow bath 36, and the other end of the platingsolution circulation line 46 is coupled to the bottom of the cathodechamber 12. The plating solution circulation line 46 is provided with apump 38, a heat exchanger (temperature regulator) 40, a filter 42, and aflow meter 44. The plating solution supply line 20 extending from theplating solution supply source 18 is coupled to the top of the overflowbath 36.

A regulation plate 50 for regulating a distribution of electricpotential in the cathode chamber 12 is disposed in the cathode chamber12. This regulation plate 50 is located between the substrate holder 22,disposed in the cathode chamber 12, and the Sn anode 32. In thisembodiment, the regulation plate 50 is made of vinyl chloride, which isa dielectric material, and has a central hole 50 a having such a size asto sufficiently restrict spreading of an electric field. A lower end ofthe regulation plate 50 reaches the bottom plate of the cathode chamber12.

A vertically-extending agitating paddle 52 serving as an agitating toolis disposed in the cathode chamber 12 at a position between thesubstrate holder 22, disposed in the cathode chamber 12, and theregulation plate 50. This agitating paddle 52 is configured to make areciprocating movement parallel to the substrate W so as to agitate theplating solution Q that exists between the substrate holder 22 and theregulation plate 50. By agitating the plating solution Q with theagitating paddle (agitating tool) 52 during plating, a sufficient amountof metal ions can be supplied uniformly to the surface of the substrateW.

An anion exchange membrane 54 is incorporated in a cathode-chamber-sidewall 10 a of the anode bath 10 which divides the interior of the platingbath 16 into the cathode chamber 12 and the anode chamber 14. Thecathode chamber 12 and the anode chamber 14 are isolated by the anionexchange membrane 54. A commercially-available product AAV manufacturedby AGC Engineering Co., Ltd., for example, can be used as the anionexchange membrane 54. The number of anion exchange membranes 54 andtheir arrangement may be arbitrarily adjusted depending on the necessarymembrane area and an amount of permeation of water molecules, which willbe described later. The anion exchange membrane 54 is incorporated intothe wall 10 a in a liquid-tight manner, e.g., by use of an O-ring sothat the plating solution Q in the cathode chamber 12 will not enter theanode chamber 14.

The wall 10 a and the anion exchange membranes 54 are arranged betweenthe Sn anode 32 and the substrate W. The wall 10 a functions as anoverflow weir which stems the anolyte E in the anode chamber 14 andallows the anolyte E to overflow the top of the wall 10 a into thecathode chamber 12. Specifically, the anolyte E is stemmed by the wall(overflow weir) 10 a and stored in the anode chamber 14 at apredetermined liquid level H (see FIG. 9). After the liquid level H isreached, the anolyte E overflows the top of the wall 10 a into the anodechamber 14.

A plating solution supply pipe 64 for supplying the plating solution Qto a dialysis cell 62, which has an anion exchange membrane 60 therein,is coupled to the plating solution circulation line 46. This platingsolution supply pipe 64 is located downstream of the flow meter 44. Aplating solution discharge pipe 66, extending from the dialysis cell 62,is coupled to a top of the overflow bath 36. The plating solution supplypipe 64 and the plating solution discharge pipe 66 constitute a platingsolution dialysis line 68 that is coupled to the plating solutioncirculation line 46 and takes in a part of the plating solution Q fromthe plating solution circulation line 46 to cause the plating solution Qto circulate therethrough. A pure water supply line 70 for supplyingpure water into the dialysis cell 62 and a pure water drainage line 72for discharging the pure water from the dialysis cell 62 are coupled tothe dialysis cell 62.

The plating solution Q, flowing through the plating solution dialysisline 68, is supplied into the dialysis cell 42, where at least a part ofthe methanesulfonic acid as a free acid is removed by dialysis using theanion exchange membrane 60. The plating solution Q after dialysis isreturned to the overflow bath 36. The methanesulfonic acid that has beenremoved from the plating solution Q by the dialysis diffuses into thepure water supplied into the dialysis cell 62 through the pure watersupply line 70, and is discharged to the exterior of the dialysis cell62 through the pure water drainage line 72.

The anion exchange membrane 60 used in this embodiment is DSVmanufactured by AGC Engineering Co., Ltd. An arbitrary number of anionexchange membranes 60 may be incorporated in the dialysis cell 62depending on the amount of the plating solution to be dialyzed (i.e.,the amount of the methanesulfonic acid to be removed).

In this embodiment, at least a part of the methanesulfonic acid as afree acid in the plating solution Q is removed by using the dialysiscell 62 that employs the diffusion dialysis. It is also possible toremove at least a part of the methanesulfonic acid from the platingsolution Q by using a free-acid removal cell that employselectrodialysis or an ion-exchange resin method.

The plating solution circulation line 46 is provided with an Sn ionconcentration measuring device 74 for measuring the Sn ion concentrationof the plating solution Q flowing through the plating solutioncirculation line 46. The plating solution circulation line 46 is furtherprovided with a methanesulfonic acid concentration measuring device 76for measuring the methanesulfonic acid concentration of the platingsolution Q flowing through the plating solution circulation line 46. Theoutput of the Sn ion concentration measuring device 74 and the output ofthe methanesulfonic acid concentration measuring device 76 (i.e.,concentration measurement values) are inputted into the plating solutionsupply source 18 and a controller 80.

FIG. 2 is a perspective view of the anode bath 10. As shown in FIG. 2,in an off-centered position at the top of the wall 10 a of the anodebath 10 that functions as the overflow weir, there is provided a cutoutportion 10 b which serves as an outlet for allowing the anolyte E tooverflow the anode chamber 14. The liquid level H (see FIG. 9) of theanolyte E held in the anode chamber 14 is determined by the position ofa lower end of the cutout portion 10 b.

The electrolytic solution supply line 24 extends downward along the sideof the anode bath 10. The electrolytic solution supply line 24 has atits lower end an electrolytic solution supply outlet 24 a for supplyingthe electrolytic solution (aqueous methanesulfonic acid solution) intothe anode chamber 14. This electrolytic solution supply outlet 24 areaches the bottom of the anode bath 10 and opens in a horizontaldirection. Similarly, the pure water supply line 26 extends downwardalong the side of the anode bath 10. The pure water supply line 26 hasat its lower end a pure water supply outlet 26 a for supplying purewater into the anode chamber 14. This pure water supply outlet 26 areaches the bottom of the anode bath 10 and opens in a horizontaldirection. The electrolytic solution supply outlet 24 a and the purewater supply outlet 26 a may open in a downward direction.

When the anode bath 10 is projected onto a horizontal plane, theelectrolytic solution supply outlet 24 a and the pure water supplyoutlet 26 a are diagonally opposite to the cutout portion 10 b of thewall 10 a so that when the pure water or the electrolytic solution issupplied into the anode chamber 14 through the pure water supply line 26or the electrolytic solution supply line 24, the anolyte E containing Snions is agitated sufficiently by the supplied pure water or electrolyticsolution and then overflows the cutout portion 10 b into the cathodechamber 12.

The N₂ gas supply line 33 extends downward along the side of the anodebath 10 to reach the bottom of the anode bath 10, and further extendshorizontally over approximately the entire length of the anode bath 10in its longitudinal direction. Nitrogen gas is released or ejectedupward through jet orifices 33 a, which are provided in the N₂ gassupply line 33, to cause the anolyte E to bubble, thereby sufficientlyagitating the anolyte E in the anode chamber 14. The bubbles of thenitrogen gas can promote uniform distribution of the Sn ions and themethanesulfonic acid throughout the anolyte E in the anode chamber 14and, in addition, can prevent oxidation of the Sn ions in the anolyte E.In view of this, the nitrogen gas is preferably supplied into theanolyte E at the bottom of the anode chamber 14 to cause the bubbling ofthe anolyte E from the bottom of the anode chamber 14.

It is preferred to stop the supply of the nitrogen gas immediatelybefore the pure water or the electrolytic solution is supplied into theanode chamber 14 so as not to carry out the bubbling of the anolyte Ewith the nitrogen gas during the supply of the pure water or theelectrolytic solution. This enables the anolyte E, containing Sn ions ina sufficiently dispersed state, to overflow the wall 10 a into thecathode chamber 12 while preventing the anolyte E from being excessivelydiluted with the pure water or the electrolytic solution supplied.

In order to detect a decrease in the amount of the anolyte E in theanode chamber 14 due to its evaporation, a liquid level detection sensor82 for detecting the liquid level of the anolyte E in the anode chamber14 is provided above the anode chamber 14. Upon detection of thedecrease in the amount of the anolyte E due to its evaporation, the purewater may be supplied into the anolyte E in the anode chamber 14 throughthe pure water supply line 26. This makes it possible to keep theanolyte E in the anode chamber 14 at a constant liquid level. Further,it is possible to control the amount of Sn ions to be supplied to thecathode chamber 12 with the amount of the pure water or the electrolyticsolution to be supplied into the anode chamber 14.

A mechanical structure may be used to cause the anolyte E in the anodechamber 14 to overflow into the cathode chamber 12. For example, asshown in FIG. 3, a float 84 may be put on the anolyte E in the anodechamber 14 and may be submerged into the anolyte E so as to cause theanolyte E to overflow into the cathode chamber 12 with an amountcorresponding to a volume of the float 84. This structure involves nosupply of pure water or the electrolytic solution, and thus nointroduction of water into the anolyte E. Therefore, the anolyte E canbe supplied into the cathode chamber 12 without dilution of the anolyteE.

As shown in FIG. 4, a vertically movable weir 86 may be provided in arectangular cutout portion 10 c that is formed in the top of the wall 10a that serves as the overflow weir. In this example shown in FIG. 4, theanolyte E can be supplied into the cathode chamber 12 by lowering themovable weir 86. This structure also has the advantage of no dilution ofthe anolyte E when supplied into the cathode chamber 12.

In the event of a shortage of Sn ions in the entire system, it isnecessary to replenish the plating solution Q with Sn ions. Aconceivable Sn ion replenishing method is to supply an Sn replenishingsolution having a high Sn ion concentration to the plating solution Q.However, such a high-concentration Sn replenishing solution is generallyexpensive, and therefore incurs high costs. Thus, in this embodiment, anauxiliary electrolytic cell 100 for replenishment of the Sn ions isprovided separately from the plating bath 16.

A box-shaped cathode bath 102 is disposed in the auxiliary electrolyticcell 100, whereby the interior of the auxiliary electrolytic cell 100 isdivided into an anode chamber (i.e., auxiliary anode chamber) 104 and acathode chamber (i.e., auxiliary cathode chamber) 106 defined in thecathode bath 102. An anion exchange membrane 108 is incorporated in ananode-chamber-side wall 102 a of the cathode bath 102 which divides theinterior of the auxiliary electrolytic cell 100 into the anode chamber104 and the cathode chamber 106. The anode chamber 104 and the cathodechamber 106 are isolated by the anion exchange membrane 108.

An anolyte supply line 110 for supplying an anolyte A containing Sn ionsand methanesulfonic acid and not containing Ag ions, and an electrolyticsolution supply line 112 for supplying an electrolytic solutioncomprising an aqueous solution containing methanesulfonic acid (i.e.,aqueous methanesulfonic acid solution) are coupled to the anode chamber104. An Sn anode (i.e., auxiliary Sn anode) 118, which is held by ananode holder 116, is disposed in the anode chamber 104 and immersed inthe anolyte A. One end of an Sn ion replenishing line 114 is coupled tothe anode chamber 104, and the other end of the Sn ion replenishing line114 is coupled to the top of the overflow bath 36 of the plating bath16. The Sn ion replenishing line 114 is provided with a pump 120.

A catholyte supply line 122 for supplying a catholyte B comprising anaqueous solution containing methanesulfonic acid (i.e., aqueousmethanesulfonic acid solution), and a liquid discharge line 124 fordischarging the catholyte B are coupled to the cathode chamber 106. Acathode (i.e., an auxiliary cathode) 128, which is made of e.g. SUS andheld by a cathode holder 126, is disposed in the cathode chamber 106 andimmersed in the catholyte B. The above-described wall 102 a and theanion exchange membrane 108 are located between the Sn anode 118 and thecathode 128.

In operation of the auxiliary electrolytic cell 100, the anolyte A,containing Sn ions at a high concentration (e.g. 220 g/L to 350 g/L) andmethanesulfonic acid and not containing Ag ions, is supplied into theanode chamber 104 through the anolyte supply line 110, thereby immersingthe Sn anode 118 in the anolyte A. The catholyte B containing an aqueousmethanesulfonic acid solution is supplied into the cathode chamber 106through the catholyte supply line 122, thereby immersing the cathode 128in the catholyte B.

In this state, a positive pole and a negative pole of an auxiliary powersource 130 are electrically connected to the Sn anode 118 and thecathode 128, respectively, to start electrolysis. Once the electrolysisis started, the Sn ion concentration of the anolyte A increases as aresult of the dissolution of Sn ions from the Sn anode 118. Because theanode chamber 104 and the cathode chamber 106 are isolated by the anionexchange membrane 108, the Sn ions do not migrate into the cathodechamber 106 and therefore the cathode 128 is not plated. Because theanolyte A does not contain Ag ions, displacement deposition of Ag on thesurface of the Sn anode 118 does not occur. The Sn ions in the anolyte Aare supplied through the anolyte supply line 110 before the start ofelectrolysis, while the Sn ions are supplied by the dissolution from theSn anode 118 after the start of electrolysis.

After a predetermined Sn ion concentration is reached in the anolyte A,the pump 120 is driven to supply the anolyte A into the overflow bath 36of the plating bath 16 through the Sn ion replenishing line 114. Theamount of the anolyte A in the anode chamber 104 decreases as a resultof the supply of the anolyte A to the overflow bath 36. Thus, theelectrolytic solution, in an amount that compensates for the decrease inthe amount of the anolyte A, is supplied into the anode chamber 104through the electrolytic solution supply line 112. The Sn ionconcentration of the anolyte A is preferably as high as possible fromthe viewpoint of decreasing the amount of waste liquid discharged fromthe entire system.

Methanesulfonate ions contained in the catholyte B in the cathodechamber 106 pass through the anion exchange membrane 108 and migrateinto the anode chamber 104. Accordingly, the conductivity of thecatholyte B in the cathode chamber 106 decreases with time. Therefore, afresh catholyte B is supplied into the cathode chamber 106 through thecatholyte supply line 122, while discharging the catholyte B from thecathode chamber 106 to the exterior through the liquid discharge line124 so that the catholyte B does not overflow.

As shown in FIGS. 5 through 8, the substrate holder 22 includes a firstholding member 154 having a rectangular plate shape and made of e.g.,vinyl chloride, and a second holding member 158 rotatably coupled to thefirst holding member 154 through a hinge 156 which allows the secondholding member 158 to open and close with respect to the first holdingmember 154. Although in this embodiment the second holding member 158 isconfigured to be openable and closable through the hinge 156, it is alsopossible to dispose the second holding member 158 opposite to the firstholding member 154 and to move the second holding member 158 away fromand toward the first holding member 154 to thereby open and close thesecond holding member 158.

The second holding member 158 includes a base portion 160 and aring-shaped seal holder 162. The seal holder 162 is made of vinylchloride so as to enable a retaining ring 164, which will be describedlater, to slide well. An annular substrate-side sealing member 166 (seeFIGS. 7 and 8) is fixed to an upper surface of the seal holder 162. Thissubstrate-side sealing member 166 is placed in pressure contact with aperiphery of the surface of the substrate W to seal a gap between thesubstrate W and the second holding member 158 when the substrate W isheld by the substrate holder 22. An annular holder-side sealing member168 (see FIGS. 7 and 8) is fixed to a surface, facing the first holdingmember 154, of the seal holder 162. This holder-side sealing member 168is placed in pressure contact with the first holding member 154 to seala gap between the first holding member 154 and the second holding member158. The holder-side sealing member 168 is located outwardly of thesubstrate-side sealing member 166.

As shown in FIG. 8, the substrate-side sealing member 166 is sandwichedbetween the seal holder 162 and a first mounting ring 170 a which issecured to the seal holder 162 by fastening tools 169 a, such as bolts.The holder-side sealing member 168 is sandwiched between the seal holder162 and a second mounting ring 170 b which is secured to the seal holder162 by fastening tools 169 b, such as bolts.

The seal holder 162 of the second holding member 158 has a steppedportion at a periphery thereof, and the retaining ring 164 is rotatablymounted to the stepped portion through a spacer 165. The retaining ring164 is inescapably held by an outwardly projecting retaining plates 172(see FIG. 6) mounted to a side surface of the seal holder 162. Thisretaining ring 164 is made of a material (e.g., titanium) having highrigidity and excellent acid and alkali corrosion resistance and thespacer 165 is made of a material having a low friction coefficient, forexample PTFE, so that the retaining ring 164 can rotate smoothly.

Inverted L-shaped clampers 174, each having an inwardly projectingportion and located outside of the retaining ring 164, are provided onthe first holding member 154 at equal intervals along a circumferentialdirection of the retaining ring 164. The retaining ring 164 hasoutwardly projecting portions 164 b arranged along the circumferentialdirection of the retaining ring 164 at positions corresponding topositions of the dampers 174. A lower surface of the inwardly projectingportion of each damper 174 and an upper surface of each projectingportion 164 b of the retaining ring 164 are tapered in oppositedirections along the rotational direction of the retaining ring 164. Aplurality (e.g., three) of upwardly protruding dots 164 a are providedon the retaining ring 164 in predetermined positions along thecircumferential direction of the retaining ring 164. The retaining ring164 can be rotated by pushing and moving each dot 164 a from a lateraldirection by means of a rotating pin (not shown).

When the second holding member 158 is open, the substrate W is insertedinto the central portion of the first holding member 154, and the secondholding member 158 is then closed through the hinge 156. Subsequentlythe retaining ring 164 is rotated clockwise so that each projectingportion 164 b of the retaining ring 164 slides into the inwardlyprojecting portion of each damper 174. As a result, the first holdingmember 154 and the second holding member 158 are fastened to each otherand locked by engagement between the tapered surfaces of the retainingring 164 and the tapered surfaces of the dampers 174. The lock of thesecond holding member 158 can be released by rotating the retaining ring164 counterclockwise and to disengage the projecting portions 164 b ofthe retaining ring 164 from the inverted L-shaped dampers 174.

When the second holding member 158 is locked in the above-describedmanner, the downwardly-protruding portion of the substrate-side sealingmember 166 is placed in pressure contact with the periphery of thesurface of the substrate W. The substrate-side sealing member 166 ispressed uniformly against the substrate W to thereby seal the gapbetween the periphery of the surface of the substrate W and the secondholding member 158. Similarly, when the second holding member 158 islocked, the downwardly-protruding portion of the holder-side sealingmember 168 is placed in pressure contact with the surface of the firstholding member 154. The sealing holder-side sealing member 168 isuniformly pressed against the first holding member 154 to thereby sealthe gap between the first holding member 154 and the second holdingmember 158.

A pair of T-shaped holder hangers 190 are provided on end portions ofthe first holding member 154. These holder hangers 190 serve as asupport when the substrate holder 22 is transported and when thesubstrate holder 22 is held in a suspended state. A protruding portion182 is formed on the upper surface of the first holding member 154 so asto protrude in a ring shape corresponding to a size of the substrate W.The protruding portion 182 has an annular support surface 180 which isplaced in contact with the periphery of the substrate W to support thesubstrate W. The protruding portion 182 has recesses 184 arranged atpredetermined positions along a circumferential direction of theprotruding portion 182.

As shown in FIG. 6, a plurality of electrical conductors (electricalcontacts) 186 (e.g., 12 conductors as illustrated), coupled respectivelyto wires extending from external contacts (not shown) provided in theholder hanger 190, are disposed in the recesses 184 of the protrudingportion 182. When the substrate W is placed on the support surface 180of the first holding member 154, end portions of the electricalconductors 186 resiliently contact the lower portions of the electricalcontacts 188 shown in FIG. 8.

The electrical contacts 188, which are to be electrically connected tothe electrical conductors 186, are secured to the seal holder 162 of thesecond holding member 158 by fastening tools 189, such as bolts. Theelectrical contacts 188 each have a leaf spring-like contact portionlying outside the substrate-side sealing member 166 and projectinginwardly. This contact portion is springy and bends easily. When thesubstrate W is held by the first holding member 154 and the secondholding member 158, the contact portions of the electrical contacts 188make elastic contact with the peripheral surface of the substrate Wsupported on the support surface 180 of the first holding member 154.

The second holding member 158 is opened and closed by a not-shownpneumatic cylinder and by the weight of the second holding member 158itself. More specifically, the first holding member 154 has athrough-hole 154 a, and a pneumatic cylinder is provided so as to facethe through-hole 154 a. The second holding member 158 is opened byextending a piston rod of the pneumatic cylinder through thethrough-hole 154 a to push up the seal holder 162 of the second holdingmember 158. The second holding member 158 is closed by its own weightwhen the piston rod is retracted.

Next, operations of the plating apparatus according to the embodimentwill be described. The pump 38 is set in motion to circulate the platingsolution Q in the cathode chamber 12 through the plating solutioncirculation line 46 to thereby agitate the plating solution Q. In thisstate, the substrate W, held by the substrate holder 22, is put at thepredetermined position in the cathode chamber 12 and immersed in theplating solution Q. The anode chamber 14 is filled with the initialanolyte E so that the Sn anode 32 is immersed in the anolyte E.

In this state, the Sn anode 32 is electrically connected to the positivepole of the plating power source 34, and a conductive layer, such as aseed layer, formed on the surface of the substrate W is electricallyconnected to the negative pole of the plating power source 34 to startplating of the surface of the substrate W. During the plating, theagitating paddle (agitating tool) 52 reciprocates or oscillates parallelto the substrate W, as necessary, to agitate the plating solution Q inthe cathode chamber 12. At the same time, the nitrogen gas is suppliedinto the anolyte E through the N₂ gas supply line 33 to form the bubblesof the nitrogen gas in the anolyte E in the anode chamber 14.

During the plating, Sn ions dissolve from the Sn anode 32 into theanolyte E in the anode chamber 14 as shown in FIG. 9. The dissolution ofthe Sn ions occurs every time plating of a substrate is performed, andtherefore the Sn ion concentration of the anolyte E in the anode chamber14 increases. Further, the volume of the anolyte E in the anode chamber14 increases when the electrolytic solution or the pure water issupplied into the anode chamber 14 from the electrolytic solution supplyline 24 or the pure water supply line 26. When the liquid level of theanolyte E in the anode chamber 14 rises over the predetermined liquidlevel H by ΔH, the anolyte E overflows the cutout portion 10 b (see FIG.2) formed in the wall 10 a of the anode chamber 14 and flows into thecathode chamber 12 by an amount corresponding to the increase ΔH in theliquid level. Therefore, some of the Sn ions in the anode chamber 14 aresupplied into the cathode chamber 12, and can compensate for theshortage of the Sn ions that have been consumed in plating of thesubstrate W. In view of the increase in the amount of the platingsolution Q when the anolyte E is supplied to the plating solution Q, theplating solution Q is discharged in advance by an amount correspondingto the amount of the anolyte E supplied into the cathode chamber 12.

When an electric field is formed between the Sn anode 32 and thesubstrate W as a cathode, the methanesulfonic acid in the cathodechamber 12, together with water molecules, passes through the anionexchange membrane 54 into the anode chamber 14. This migration alsoincreases the amount of the anolyte E in the anode chamber 14, and as aresult the anolyte E overflows the wall 10 a into the cathode chamber 12by an amount exceeding the predetermined liquid level H. In this manner,the Sn ions in the anode chamber 14 can be supplied into the cathodechamber 12.

The present inventors have verified through experiments the fact thatthe concentration of methanesulfonic acid as a free acid in the anolyteE in the anode chamber 14 is important for stabilizing the Sn ions thathave dissolved from the Sn anode. In particular, an experiment wasconducted in which an anolyte of an aqueous methanesulfonic acidsolution, initially having a methanesulfonic acid concentration of 100g/L, was supplied in an anode chamber at the start of electrolysis. Inthis case, the anolyte in the anode chamber was found to become cloudyas the electrolysis was continued. This indicates that Sn ions cannotexist stably as divalent ions in the anolyte, and precipitate as metalSn or tetravalent Sn ions are generated.

In contrast, in an experiment in which the electrolysis was started withan initial methanesulfonic acid concentration of 140 g/L, the anolyte inthe anode chamber did not become cloudy during the electrolysis.Moreover, the Sn ion concentration of the anolyte agreed with acalculation value that was determined on condition that Sn has dissolvedas divalent Sn ions. This indicates that because of the presence of asufficient amount of methanesulfonate ions in the anolyte, divalent Snions exist stably in the form of a complex surrounded bymethanesulfonate ions. As will be appreciated from the foregoing, themethanesulfonic acid concentration of the anolyte should preferably becontrolled in such a range as to allow the divalent Sn ions to existstably in the anolyte.

As described above, by supplying the pure water into the anode chamber14 through the pure water supply line 26, the anolyte E in the anodechamber 14 can overflow into the cathode chamber 12 to supply Sn ions tothe cathode chamber 12. The plating apparatus of this embodiment is alsoprovided with the electrolytic solution supply line 24 for supplying theelectrolytic solution (the aqueous methanesulfonic acid solution) intothe anode chamber 14. This is because of the following reasons.

When the pure water is supplied into the anode chamber 14 through thepure water supply line 26 to cause the anolyte E in the anode chamber 14to overflow into the cathode chamber 12, the methanesulfonic acid in theanolyte 14 flows into the cathode chamber 12, and therefore themethanesulfonic acid concentration of the anolyte E in the anode chamber14 decreases. The methanesulfonic acid in the cathode chamber 12 passesthrough the anion exchange membrane 54 and migrates into the anodechamber 14 by forming an electric field between the Sn anode 32 and thesubstrate W as a cathode. The transference number of methanesulfonicacid is not 100%, but can be 50% to 90% due to a loss, although itdepends on conditions. Thus, a ratio of the mol concentration of themethanesulfonic acid that passes through the anion exchange membrane 54into the anode chamber 14 to the mol concentration of Sn ions thatdissolve from the Sn anode 32 into the anolyte E in the anode chamber 14will deviate from 1:2. Consequently, the methanesulfonic acidconcentration of the anolyte E in the anode chamber 14 will decrease,whereby Sn ions in the anode chamber 14 may become unstable as describedabove.

It is therefore necessary to supply the electrolytic solution containingthe methanesulfonic acid into the anode chamber 14 through theelectrolytic solution supply line 24 so that the methanesulfonic acidconcentration of the anolyte E in the anode chamber 14 is not loweredbelow an acceptable value.

In order to operate the plating apparatus efficiently, it is desirableto supply the anolyte E into the cathode chamber 12 by causing theanolyte E to overflow the anode chamber 14 while keeping the Sn ionconcentration of the anolyte E in the anode chamber 14 as high aspossible. If the anolyte E with a low Sn ion concentration is suppliedinto the cathode chamber 12, a larger amount (overflow amount) of theanolyte E needs to be supplied from the anode chamber 14 in order tosupply a certain amount of Sn ions into the cathode chamber 12. As aresult, a larger amount of the plating solution Q should be dischargedfrom the circulation system including the cathode chamber 12, making theplating process uneconomical.

Specifically, the Sn ion concentration of the anolyte E in the anodechamber 14 is controlled typically in the range of 80 g/L to 500 g/L,preferably in the range of 200 g/L to 400 g/L, more preferably in therange of 220 g/L to 350 g/L. The Sn ion concentration of the anolyte Ecan be determined from the Sn ion concentration of a fresh anolyte Ewhich has been put into the anode chamber 14 before the start of platingand the Sn ion concentration converted from the quantity of electricityat the Sn anode 32 after the start of plating. The Sn ion concentrationof the anolyte E is of significant importance for controlling theconcentration of Sn ions in the entire plating bath. The Sn ionconcentration of the Sn—Ag plating solution Q is usually 50 g/L to 80g/L. When the decrease in the Sn ion concentration of the platingsolution Q in the cathode chamber 12 is to be compensated by the supplyof the anolyte E containing the Sn ion in the anode chamber 14, the useof the anolyte E with a higher Sn ion concentration can reduce itsvolume to be supplied into the cathode chamber 12. The amount of theplating solution Q in the cathode chamber 12 usually decreases due toevaporation of the solution, etc. When the anolyte E in the anodechamber 14 is supplied to the plating solution Q in the cathode chamber12 in an amount more than the decrease in the amount of the platingsolution Q, the excess amount of the plating solution Q needs to befinally discharged from the cathode chamber 12. However, the Sn ionconcentration of the anolyte E cannot be increased to a value more thana saturation concentration of tin methanesulfonate. Further, the Sn ionconcentration of the anolyte E should be kept less than the saturationconcentration in order for the Sn ions to exist stably.

The pure water supply line 26 is used not only for supplying the purewater into the anode chamber 14 when replenishing the anode chamber 14with water by an amount corresponding to the amount of evaporated water,but also for causing the anolyte E in the anode chamber 14 to overflowthe wall 10 a so as to supply the Sn ions to the cathode chamber 12 whenthe methanesulfonic acid concentration of the anolyte E in the anodechamber 14 is sufficiently high. Further, the pure water supply line 26is used for supplying the pure water into the anode chamber 14 so as toadjust the concentration of a component of the anolyte E in the anodechamber 14.

An exemplary operation of the Sn alloy plating apparatus shown in FIG. 1will now be described.

Before starting the operation of the Sn alloy plating apparatus, theanolyte E, containing Sn ions at a high concentration (e.g., 220 g/L to350 g/L) and methanesulfonic acid, is supplied into the anode chamber 14to fill the anode chamber 14 with the anolyte E. As described above, itis preferable to supply the anolyte E at a high Sn ion concentration inthe anode chamber 14 into the cathode chamber 12 because the amount ofthe plating solution Q to be discharged as waste can be reduced. If theoperation of the apparatus is started with the anolyte E at a low Snconcentration, it is necessary to wait to supply the anolyte E into thecathode chamber 12 until a high Sn ion concentration of the anolyte E isreached.

The pump 38 is actuated to circulate the plating solution Q in thecathode chamber 12 through the plating solution circulation line 46,thereby agitating the plating solution Q in the cathode chamber 12. Inthis state, a substrate W, which is held by the substrate holder 22, isput at a predetermined position in the cathode chamber 12 and immersedin the plating solution Q.

The Sn anode 32 is electrically connected to the positive pole of theplating power source 34, and a conductive layer, such as a seed layer,formed on the surface of the substrate W is electrically connected tothe negative pole of the plating power source 34 to start plating of thesurface of the substrate W. During the plating, the agitating paddle(agitating tool) 52 is caused to make a reciprocating movement parallelto the substrate W, as necessary, so as to agitate the plating solutionQ in the cathode chamber 12. At the same time, the nitrogen gas issupplied into the anolyte E in the anode chamber 14 through the N₂ gassupply line 33 to form nitrogen gas bubbles in the anolyte E.

While the plating of the substrate W is performed in this manner, the Snion concentration of the plating solution Q is measured by the Sn ionconcentration measuring device 74, and a signal of the measurementresults (i.e., a measurement value) is sent to the controller 80. Inthis embodiment, the controller 80 estimates the methanesulfonic acidconcentration of the anolyte E in the anode chamber 14 and, based on theestimated value, determines whether to supply the electrolytic solutioninto the anode chamber 14 through the electrolytic solution supply line24 or to supply the pure water into the anode chamber 14 through thepure water supply line 26, or to supply both the electrolytic solutionand the pure water. Specifically, when the concentration ofmethanesulfonic acid as a free acid in the anolyte E has been reducedbelow a predetermined value, the electrolytic solution, containing themethanesulfonic acid, is supplied into the anode chamber 14 through theelectrolytic solution supply line 24 so that the methanesulfonic acidconcentration of the anolyte E does not become lower than a lower limitvalue. When replenishing the plating solution Q in the cathode chamber12 with Sn ions through the supply of the anolyte E having asufficiently high methanesulfonic acid concentration, the pure water issupplied into the anode chamber 14 through the pure water supply line26. The supply of the pure water into the anode chamber 14 causes theanolyte E to overflow into the cathode chamber 12, thereby supplying Snions to the plating solution Q in the cathode chamber 12.

The concentration of methanesulfonic acid as a free acid contained inthe anolyte E in the anode chamber 14 is controlled to be not less than30 g/L, so that the Sn ions at a high concentration, e.g., 220 g/L to350 g/L, can exist stably as divalent ions. When the methanesulfonicacid concentration of the anolyte E is high, the supply of the anolyte Eto the plating solution Q can appreciably increase the methanesulfonicacid concentration of the plating solution Q in the cathode chamber 12,which may result in poor film-thickness uniformity in the platingprocess as will be described later. Therefore, the methanesulfonic acidconcentration of the plating solution Q is controlled so as not toexceed a particular value which is determined by taking the actualoperating conditions of the apparatus into consideration.

The concentration of methanesulfonic acid as a free acid in the platingsolution Q in the cathode chamber 12 varies with the quantity ofelectricity and the current efficiency at the Sn anode 32, the amount ofthe anolyte E that has overflowed into the plating solution Q, theamount of waste liquid (drain-out) discharged from the plating solutioncirculation line 46, and the permeability of the anion exchange membrane54 with respect to the methanesulfonic acid. The film-thicknessuniformity in plating of the substrate tends to be poor when themethanesulfonic acid concentration of the plating solution Q in thecathode chamber 12 exceeds about 250 g/L. Therefore, when themethanesulfonic acid concentration measuring device 76 detects that themethanesulfonic acid concentration of the plating solution Q in thecathode chamber 12 exceeds an upper limit value, the plating solution Qis forced to flow into the plating solution dialysis line 68 having thedialysis cell 62, which removes the methanesulfonic acid from theplating solution Q. The plating solution Q, from which themethanesulfonic acid has been removed, is returned to the overflow bath36. The dialysis of the plating solution Q in the dialysis cell 62 canadjust the methanesulfonic acid concentration of the plating solution Qpreferably in the range of 60 g/L to 250 g/L, more preferably in therange of 90 g/L to 150 g/L.

The concentration of the methanesulfonic acid as a free acid in theanolyte E during operation of the Sn alloy plating apparatus may becontrolled based on an estimated value of the methanesulfonic acidconcentration of the anolyte E in the anode chamber 14. This estimatedvalue of the methanesulfonic acid concentration can be determinedtheoretically or experimentally from an initial methanesulfonic acidconcentration of the anolyte E, the quantity of electricity and thecurrent efficiency at the Sn anode 32, the amount of the electrolyticsolution supplied through the electrolytic solution supply line 24, theamount of pure water supplied through the pure water supply line 26, andthe permeability of the anion exchange membrane 54 with respect to themethanesulfonic acid that passes through the anion exchange membrane 54and migrates from the cathode chamber 12 into the anode chamber 14. TheSn ion concentration and the methanesulfonic acid concentration of theanolyte E in the anode chamber 14 can be estimated from a curve of theamount of dissolved Sn ions associated with the quantity of electricityduring plating and from the permeability of the anion exchange membranewith respect to the acid.

As described above, before starting the operation of the Sn alloyplating apparatus, the anolyte E, containing Sn ions at a highconcentration (e.g., 220 g/L to 350 g/L) and methanesulfonic acid, issupplied into the anode chamber 14. When the Sn ion concentration of theanolyte E in the anode chamber 14, as estimated e.g. from the quantityof electricity at the Sn anode and the efficiency of electrolysis,reaches a predetermined threshold value (e.g., 300 g/L) during operationof the Sn alloy plating apparatus, the electrolytic solution is suppliedinto the anode chamber 14 through the electrolytic solution supply line24 to cause the anolyte E to overflow the wall 10 a, therebyreplenishing the plating solution Q in the cathode chamber 12 with Snions.

Although the Sn ion concentration of the anolyte E in the anode chamber14 decreases as a result of the supply of the electrolytic solution, theSn ion concentration increases gradually during plating and eventuallyreaches the threshold value. During this plating process, Sn ions in theplating solution Q are consumed in plating of the substrate W. Assumingthat the efficiency of electrolysis at the substrate W is equal to theefficiency of electrolysis at the Sn anode 32 and that no Sn ions aredischarged out of the system, Sn ions will dissolve from the Sn anode 32in an amount equal to the amount of Sn ions consumed in plating of thesubstrate W. Thus, the amount of Sn ions in the entire system is keptconstant. In fact, however, the efficiency of electrolysis decreaseswith the increase in the Sn ion concentration of the anolyte E in theanode chamber 14. Accordingly, the amount of Sn ions that are suppliedto the anolyte E by the dissolution from the Sn anode 32 becomes smallerthan the amount of Sn ions consumed in plating, resulting in a shortageof Sn ions in the entire system.

FIG. 10 is a graph showing the theoretical Sn ion concentration of theanolyte E in the anode chamber 14, calculated from the quantity ofelectricity, in comparison with the actually measured Sn ionconcentration of the anolyte E. As can be seen in FIG. 10, while theefficiency of electrolysis is approximately 100% when the Sn ionconcentration of the anolyte E in the anode chamber 14 is not more thanabout 130 g/L, the electrolysis efficiency decreases when the Sn ionconcentration is more than about 150 g/L, and the electrolysisefficiency decreases to about 80% at an Sn ion concentration of 300 g/L.The data shown in FIG. 10 thus indicates that when it is intended tocontrol the Sn ion concentration of the anolyte E at a high level as 220g/L to 350 g/L, 10% to 20% of Sn ions will be in short supply in theentire system. It is also noted that since the anolyte E in the anodechamber 14 overflows into the cathode chamber 12, the plating solution Qcontaining the Sn ions in the cathode chamber 12 or the overflow bath 36is discharged in advance, resulting in the shortage of the amount of Snions in the entire system.

Thus, the Sn alloy plating apparatus of this embodiment includes theauxiliary electrolytic cell 100 for compensating for the shortage of theSn ions in the entire system. The electrolysis operation of theauxiliary electrolytic cell 100 is started simultaneously with the startof operation of the Sn alloy plating apparatus or at an appropriatetime. The pump 120 is driven based on the concentration of Sn ionsmeasured by the Sn ion concentration measuring device 74 to therebysupply the anolyte A having a high Sn ion concentration in the anodechamber 104 to the overflow bath 36 of the plating bath 16. The supplyof Sn ions from the auxiliary electrolytic cell 100 can compensate forthe shortage of Sn ions caused by the difference between theelectrolytic efficiency of plating on the substrate W and the efficiencyof electrolysis at the Sn anode 32 in the anode chamber 14 and by thedischarge of the plating solution Q from the plating bath 16.

When the Sn alloy plating apparatus is operated over a long period oftime, the Sn ion concentration and the methanesulfonic acidconcentration of the anolyte E in the anode chamber 14 may deviate fromthe estimated concentrations. Therefore, the Sn concentration and themethanesulfonic acid concentration of the plating solution Q aremeasured by the Sn ion concentration measuring device 74 and themethanesulfonic acid concentration measuring device 76, and theirchanges are recorded. If the Sn ion concentration tends to become higheror lower than a concentration as estimated from the operatingconditions, then the efficiency of Sn ion dissolution, which is used forthe estimation of the concentration, will be changed. If themethanesulfonic acid concentration tends to become higher or lower thanan estimated concentration, then the permeability of the anion exchangemembrane with respect to the acid will be changed. After changing such afactor(s), control of the Sn concentration and the methanesulfonic acidconcentration is continued.

The supply of the anolyte E, containing a high concentration of Sn ions,from the anode chamber 14 to the cathode chamber 12 is preferablyperformed by forcing the anolyte E to overflow the anode chamber 14,rather than by passing the anolyte E through a pipe using a dedicatedpump. This is because of the following reasons.

If the anolyte E containing Sn ions with a high concentration resides ina pipe for a long time, deposition of a metal (which is abnormaldeposition) on an interior surface of the pipe will occur even when thesurface of the pipe is made of an insulating material. Once the metalbegins to deposit on the interior surface of the pipe, the metal tendsto grow continuously on the surface. If the supply of the anolyte E fromthe anode chamber 14 to the cathode chamber 12 is continued in order topass the anolyte E continuously through the pipe, then the total amountof the liquid in the cathode chamber increases. As a result, it isnecessary to continuously discharge the plating solution Q from thecathode chamber by the same amount as the amount of the anolyte Esupplied.

The above-described metal deposition in the pipe can be avoided by usingthe overflow method to supply the anolyte E. The anolyte E in the anodechamber 14 is constantly agitated by bubbling thereof with the supply ofthe nitrogen gas. This can prevent deposition of a metal on the innersurface of the anode chamber 14. In the embodiment, the anolyte Eoverflows the anode chamber 14 as a result of the migration ofmethanesulfonic acid and water molecules caused by the electrolysis. Theamount or volume of the anolyte E overflowing into the cathode chamber12 is exactly equal to the amount or volume of the methanesulfonic acidand the water that have passed through the anion exchange membrane 54.Thus, there is no change in the volume of the plating solution Q in thecathode chamber 12, and therefore there is no need to discharge theplating solution Q.

FIG. 11 schematically shows a plating bath 16 a which is anotherexample. An anode holder 30, holding a disk-shaped Sn anode 32, isdisposed in the anode chamber 14 of the plating bath 16 a. An annularanode mask 200 for restricting a contact area of the Sn anode 32 withanolyte E is mounted to a front surface of the anode holder 30 in amanner such that the annular anode mask 200 hermetically contact aperipheral area of the Sn anode 32. An opening 10 d is formed in thecathode-chamber-side wall 10 a of the anode bath 10. Anion exchangemembrane 54 is mounted to the wall 10 a along the edge of the opening 10d, with its peripheral portion held between a mask member 202 and thewall 10 a. The mask member 202 is provided for restricting a contactarea of the anion exchange membrane 54 with the plating solution Q.Since the wall 10 a and the mask member 202 hold the anion exchangemembrane 54 therebetween to seal a gap along the peripheral portion ofthe anion exchange membrane 54, a liquid leakage between the cathodechamber 12 and the anode chamber 14 can be prevented.

The anion exchange membrane 54 and the opening 10 d may have arectangular shape, and the mask member 202 may be a rectangular ring.The opening sizes of the opening 10 d and the mask member 202 may beequal to or larger than the inner diameter of the anode mask 200. Inorder to reduce an overall resistance between the anode and the cathode,the anion exchange membrane 54 may preferably contact the anolyte E orthe plating solution Q at an area larger than an area at which the Snanode 32 contacts the anolyte E.

An electric field shield 204, having approximately the same externalshape as that of the mask member 202 and having an opening 204 a of acircular shape similar to the shape of the substrate W, is mounted tothe front surface of the mask member 202. The diameter of the opening204 a is smaller than the opening size of the mask member 202. Theelectric field shield 204, which is provided in the cathode chamber 12at a position near the Sn anode 32, can reduce a thickness of a seedlayer formed on the substrate, making it possible to make thedistribution of the film thickness uniform even in a case where the filmthickness would otherwise be relatively large in a peripheral area ofthe substrate. The electric field shield 204 may have a mechanism tochange its opening area in order to control the film-thicknessdistribution. The diameter of the opening 204 a of the electric fieldshield 204 is set equal to or smaller than the diameter of the centralhole 50 a of the regulation plate 50 which is located between thesubstrate W and the Sn anode 32. In this embodiment, the regulationplate 50 includes a plate 206 and a cylindrical member 208 mounted tothe plate 206.

When the anolyte E in the anode chamber 14 overflows the wall 10 a andis supplied into the cathode chamber 12, not only the Sn ions butunnecessary water is supplied as well, resulting in a considerableincrease in the amount of the plating solution Q in the cathode chamber12 and the overflow bath 36. When the amount of the plating solution Qexceeds a predetermined value, the excess solution must be discharged,leading to increased costs. In order to avoid such an issue, the Snalloy plating apparatus of this embodiment has a gas supply unit 210,which is disposed above the plating bath 16 a, for promoting evaporationof water. The gas supply unit 210 can evaporate water in the cathodechamber 12 with the same amount as the amount of the anolyte E suppliedfrom the anode chamber 14. This makes it possible to stably keep theconcentrations of the components of the plating solution Q in thecathode chamber 12, thereby eliminating the need of discharging theplating solution Q or reducing the amount of the plating solution Q tobe discharged.

In order to further reduce the amount of the plating solution to bedischarged, the plating solution circulation line 46 may be providedwith a dewatering device, which can remove only water, so that theplating solution Q passes through the dewatering device.

FIG. 12 is a schematic view of the Sn alloy plating apparatus accordingto another embodiment. This embodiment differs from the embodimentillustrated in FIG. 1 in that the plating bath 16 b of this embodimentincludes an inner bath 220 which is integral with the anode bath 10, andoverflow bath 36 provided around the inner bath 220, and that a wall 10e, which is adjacent to the overflow bath 36, of the anode bath 10functions as an overflow weir which stems the anolyte E in the anodechamber 14 and allows the anolyte E to overflow its top into theoverflow bath 36. Thus, the anolyte E is stemmed by the wall (overflowweir) 10 e and held in the anode chamber 14 at a predetermined liquidlevel H (see FIG. 9). After the liquid level H is reached, the anolyte Eoverflows the top of the wall 10 e and flows into the overflow bath 36surrounding the plating bath 16 b. Sn ions, which have been thus fedinto the overflow bath 36, are supplied into the cathode chamber 12 viathe plating solution circulation line 46.

FIG. 13 is a schematic view of the Sn alloy plating apparatus accordingto yet another embodiment. This embodiment differs from the embodimentillustrated in FIG. 1 in that the anode bath 10 of this embodiment isprovided with an anolyte circulation line 230 for drawing out a part ofthe anolyte in the anode chamber 14 from the bottom of the anode bath 10and returning the anolyte to the top of the anode bath 10. The anolytecirculation line 230 is provided with a pump 232 and a methanesulfonicacid concentration measuring device 234.

According to this embodiment, the pump 232 is driven to circulate theanolyte E in the anode chamber 14 through the anolyte circulation line230, while the methanesulfonic acid concentration measuring device 234can measure the methanesulfonic acid concentration of the anolyte Econtinually or periodically.

FIG. 14 is a schematic view of the Sn alloy plating apparatus accordingto yet another embodiment. This embodiment differs from the embodimentillustrated in FIG. 1 in that the liquid discharge line 28 of theplating bath 16 and the electrolytic solution supply line 112 of theauxiliary electrolytic cell 100, shown in FIG. 1, are coupled by aconnection line 242 which is provided with a pump 240, and that the Snion replenishing line 114, extending form the anode chamber 104 of theauxiliary electrolytic cell 100, is coupled to the top of the anodechamber 14 of the plating bath 16.

According to this embodiment, the anolyte E in the anode chamber 14 ofthe plating bath 16 can be used as an electrolytic solution to besupplied to the anode chamber 104 of the auxiliary electrolytic cell100, while the anolyte A having a high Sn ion concentration in the anodechamber 104 of the auxiliary electrolytic cell 100 can be returned tothe anode chamber 14 of the plating bath 16. The circulating anolyte cancompensate for the shortage of Sn ions in the plating system.

FIG. 15 is a schematic view of the Sn alloy plating apparatus having aplurality of plating baths, according to yet another embodiment. Asshown in FIG. 15, the Sn alloy plating apparatus includes a plurality ofplating baths 250, each having the same construction as the plating bath16 shown in FIG. 1, and a single reservoir bath 252. The anode chambersof the respective plating baths 250 coupled to the reservoir bath 252 byan anolyte supply line 254 and an anolyte recovery line 256. The anolytesupply line 254 is provided with a pump 258 a. The anolyte supply line254 branches into branch lines extending to the plating baths 250,respectively. Branch points of the anolyte supply line 254 are locateddownstream of the pump 258 a. Switching valves 260 a are provided at thebranch points of the anolyte supply line 254. The anolyte recovery line256 is provided with a pump 258 b. The anolyte recovery line 256branches into branch lines extending to the plating baths 250,respectively. Branch points of the anolyte recovery line 256 are locatedupstream of the pump 258 b. Switching valves 260 b are provided at thebranch points of the anolyte recovery line 256.

A heater 262 for heating the anolyte is installed in the reservoir bath252 in order to raise the temperature of the anolyte so as to increasethe efficiency of electrolysis. The temperature of the anolyte iscontrolled e.g., in the range of 26° C. to 40° C.

According to this embodiment, the Sn ion concentration and themethanesulfonic acid concentration of the anolyte can be made equal inall the anode chambers of the plating baths 250 by circulating theanolyte between the anode chambers of the plating baths 250 and thereservoir bath 252. Thus, control of the Sn ion concentration and themethanesulfonic acid concentration of the anolyte can be performed in aconsiderably simple manner according to this embodiment as compared tothe case of controlling these concentrations of the anolyte individuallyin each of the plating baths 250.

In this embodiment, the anolyte circulates between the reservoir bath252 and one of the plating baths 250 by using the two pumps 258 a, 258 band operating the switching valves 260 a, 260 b. This enables easycontrol of the anolyte in the anode chamber of each plating bath 250.Pumps may be provided for the plating baths 250, respectively, in orderto circulate the anolyte between the anode chambers of the plating baths250 and the reservoir bath 252. Thus, the circulation of the anolytebetween one plating bath 250 and the reservoir bath 252 may be performedindependently of the other baths 250.

In order to eliminate the shortage of Sn ions caused by the differencebetween the electrolytic efficiency of the substrate plating and theelectrolytic efficiency at the Sn anode in each anode chamber and by thedischarge of the plating solution from each plating bath, the reservoirbath 252 may be provided with an auxiliary electrolytic cell, having thesame construction as the auxiliary electrolytic cell 100 shown in FIG.1, so as to compensate for the shortage of the Sn ions.

In yet another embodiment, the Sn alloy plating apparatus may includeone outer bath (overflow bath) and a plurality of cathode chambers. Ananolyte is supplied from the outer bath into each cathode chamber from abottom of each cathode chamber by means of a pump, and the liquid in thecathode chamber is returned by overflow to the outer bath. Thisconfiguration enables easy control of the liquid in the cathodechambers.

While the present invention has been described with reference topreferred embodiments, it is understood that the present invention isnot limited to the embodiments described above, but is capable ofvarious changes and modifications within the scope of the inventiveconcept as expressed herein.

What is claimed is:
 1. An Sn alloy plating apparatus forelectrodepositing an alloy of Sn and a metal which is nobler than Sn ona surface of a substrate, the apparatus comprising: a plating bath whoseinterior is separated by an anion exchange membrane into a cathodechamber for holding therein an Sn alloy plating solution in which thesubstrate, serving as a cathode, is to be immersed and an anode chamberfor holding therein an anolyte containing Sn ions and an acid that formsa complex with a divalent Sn ion; an Sn anode located in the anodechamber; and an electrolytic solution supply line configured to supplyan electrolytic solution containing the acid into the anode chamber suchthat a Sn ion concentration of the anolyte in the anode chamber is keptnot less than a predetermined value and a concentration of the acid inthe anolyte is kept not less than a predetermined acceptable value, theelectrolytic solution supply line being configured to supply theelectrolytic solution into the anode chamber to increase an amount ofthe anolyte in the anode chamber and supply the anolyte into the Snalloy plating solution by the increased amount.
 2. The Sn alloy platingapparatus according to claim 1, wherein the electrolytic solution supplyline is configured to supply the electrolytic solution into the anodechamber to increase an amount of the anolyte in the anode chamber tothereby cause the anolyte to overflow the anode chamber into the Snalloy plating solution.
 3. The Sn alloy plating apparatus according toclaim 1, further comprising: an overflow bath configured to store the Snalloy plating solution that has overflowed the cathode chamber; and aplating solution circulation line configured to return the Sn alloyplating solution in the overflow bath to the cathode chamber to therebycirculate the Sn alloy plating solution.
 4. The Sn alloy platingapparatus according to claim 1, further comprising: a pure water supplyline configured to supply pure water into the anode chamber.
 5. The Snalloy plating apparatus according to claim 1, further comprising: anacid concentration measuring device configured to measure theconcentration of the acid in the anolyte in the anode chamber.
 6. The Snalloy plating apparatus according to claim 1, further comprising: adialysis cell configured to draw out a part of the Sn alloy platingsolution from the cathode chamber, remove at least a part of the acidfrom the Sn alloy plating solution, and then return the Sn alloy platingsolution to the cathode chamber.
 7. The Sn alloy plating apparatusaccording to claim 1, further comprising: an N₂ gas supply lineconfigured to supply nitrogen gas into the anolyte in the anode chamberto form nitrogen gas bubbles in the anolyte.
 8. The Sn alloy platingapparatus according to claim 1, further comprising: an auxiliaryelectrolytic cell configured to supply an anolyte having an increasedconcentration of Sn ions to the Sn alloy plating solution, the auxiliaryelectrolytic cell including an auxiliary anode chamber for holding ananolyte therein, an auxiliary cathode chamber for holding a catholytetherein, an anion exchange membrane separating the auxiliary anodechamber and the auxiliary cathode chamber from each other, an auxiliarySn anode located in the auxiliary anode chamber, an auxiliary cathodelocated in the auxiliary cathode chamber, and an auxiliary power sourceconfigured to apply a voltage between the auxiliary Sn anode and theauxiliary cathode when the auxiliary Sn anode is immersed in the anolyteand the auxiliary cathode is immersed in the catholyte to produce theanolyte having the increased concentration of Sn ions.
 9. An Sn alloyplating method of electrodepositing an alloy of Sn and a metal which isnobler than Sn on a surface of a substrate, the method comprising:providing a plating bath whose interior is separated by an anionexchange membrane into a cathode chamber and an anode chamber; supplyingan Sn alloy plating solution into the cathode chamber; immersing thesubstrate in the Sn alloy plating solution; supplying an anolyte,containing Sn ions and an acid that forms a complex with a divalent Snion, into the anode chamber to immerse an Sn anode in the anolyte;supplying an electrolytic solution containing the acid into the anodechamber such that a Sn ion concentration of the anolyte in the anodechamber is kept not less than a predetermined value and a concentrationof the acid in the anolyte is kept not less than a predeterminedacceptable value; and applying a voltage between the Sn anode and thesubstrate serving as a cathode to plate the surface of the substrate,while supplying the electrolytic solution into the anode chamber toincrease an amount of the anolyte in the anode chamber and supplying theanolyte into the Sn alloy plating solution by the increased amount. 10.The Sn alloy plating method according to claim 9, wherein the supplyingthe electrolytic solution into the anode chamber to increase an amountof the anolyte in the anode chamber and the supplying the anolyte intothe Sn alloy plating solution by the increased amount comprisessupplying the electrolytic solution into the anode chamber to increasean amount of the anolyte in the anode chamber to thereby cause theanolyte to overflow the anode chamber into the Sn alloy platingsolution.
 11. The Sn alloy plating method according to claim 9, furthercomprising: circulating the Sn alloy plating solution in the cathodechamber.
 12. The Sn alloy plating method according to claim 9, furthercomprising: controlling an amount of the electrolytic solution or purewater to be supplied into the anode chamber based on the concentrationof the acid in the anolyte held in the anode chamber.
 13. The Sn alloyplating method according to claim 9, further comprising: determining theconcentration of the acid in the anolyte from an initial acidconcentration of the anolyte, a quantity of electricity and a currentefficiency at the Sn anode, an amount of the electrolytic solutionsupplied, and a permeability of the anion exchange membrane with respectto methanesulfonic acid that passes through the anion exchange membraneand migrates from the cathode chamber into the anode chamber.
 14. The Snalloy plating method according to claim 9, further comprising: drawingout a part of the Sn alloy plating solution from the cathode chamber;removing at least a part of the acid from the Sn alloy plating solutionthat has been drawn out; and then returning the Sn alloy platingsolution to the cathode chamber.
 15. The Sn alloy plating methodaccording to claim 9, further comprising: supplying nitrogen gas intothe anolyte in the anode chamber to form nitrogen gas bubbles in theanolyte.
 16. The Sn alloy plating method according to claim 9, furthercomprising: immersing an auxiliary Sn anode in an anolyte held in anauxiliary anode chamber; immersing an auxiliary cathode in a catholyteheld in an auxiliary cathode chamber that is separated from theauxiliary anode chamber by an anion exchange membrane; applying avoltage between the auxiliary Sn anode and the auxiliary cathode toproduce the anolyte having an increased concentration of Sn ions; andsupplying the anolyte having the increased concentration of Sn ions intothe Sn alloy plating solution.